A tool for turning or a tool for milling includes a single coated cutting insert or a plurality of coated cutting inserts. As shown in FIG. 1, such a coated cutting insert 1 includes a rake face 2 located on a side carrying chip 6 of a work material 5 in the cutting process and a flank face 3 located on a side opposed to the work material itself. Rake face 2 and flank face 3 continue to each other, with a cutting edge 4 lying therebetween. Here, cutting edge 4 serves as a central point of application for cutting work material 5.
A cutting edge of such a coated cutting insert should be changed when the life of the tool ends. Here, in the case of an insert having a single cutting edge, the insert itself should be changed. Meanwhile, a coated cutting insert having a plurality of cutting edges can be used in such a manner that its orientation is changed several times so as to use its cutting positions different from the already-used cutting position while using an identical datum plane, that is, an unused cutting edge is brought to the cutting position. In some cases, the cutting edge may be attached to another datum plane, where an unused cutting edge may be used.
At a cutting worksite, however, in spite of presence of an unused cutting edge, the coated cutting insert may be replaced or its orientation may be changed. This is because whether or not the cutting edge has been used is not recognized at the time when the cutting edge is changed or when the orientation of the cutting edge is changed. Therefore, such an operation should be performed only after whether or not the cutting edge has been used is well confirmed.
A coated cutting insert having a flank face and a rake face different in color from each other has been proposed as a method of readily recognizing a used cutting edge (Japanese Patent Laying-Open No. 2002-144108 (Patent Document 1)). Specifically, the coated cutting insert is structured such that a wear-resistant base layer called an antifriction coating layer is formed on a substrate and an indicating layer made from a material susceptible to wear is formed on the flank face.
The coated cutting insert structured as above attains an attention-drawing function indicating whether the cutting edge has been used or not, however, the indicating layer formed on the flank face tends to adhere to the work material. Specifically, the indicating layer adheres to the surface of the work material or the work material adheres to the indicating layer, with the result that an uneven cutting edge is used in the cutting process. In such a case, appearance and surface smoothness of the work material after cutting is impaired.
In addition, if the indicating layer is provided on the flank face as in this coated cutting insert, it is difficult to readily recognize which cutting edge has been used when the insert is stored in a storage case or when the insert is placed on a workbench around a machine tool. Usually, the flank face has an area smaller than the rake face, and therefore the insert is stored in the storage case or placed on the workbench with the rake face facing upward, which makes it difficult to observe the flank face.
In an attempt to solve the above-described problems, a coated cutting insert structured such that no indicating layer is formed solely around a cutting edge and a base layer is exposed in that portion has conventionally been proposed, and this coated cutting insert is expected to solve the aforementioned problems to some extent (International Publication No. 02/004156 Pamphlet (Patent Document 2)). Set aside the problem above, however, the coated cutting insert structured in such a manner has suffered the problem that chipping of the cutting edge is likely, and solving this problem has also been demanded.    Patent Document 1: Japanese Patent Laying-Open No. 2002-144108    Patent Document 2: International Publication No. 02/004156 Pamphlet